Diurnal changes in serum triglycerides as related to changes in lipolytic enzymes, (apo) lipoproteins and hormones in normal subjects on a carbohydrate-rich diet

Familial longevity is characterized by high circadian rhythmicity of serum cholesterol in healthy elderly individuals

The biological clock, whose function deteriorates with increasing age, determines bodily circadian (i.e. 24h) rhythms, including that of cholesterol metabolism. Dampening of circadian rhythms has been associated with aging and disease. Therefore, we hypothesized that individuals with a familial predisposition for longevity have a higher amplitude circadian serum cholesterol concentration rhythm. The aim of this study was to investigate circadian rhythmicity of serum cholesterol concentrations in offspring of nonagenarian siblings and their partners. Offspring from nonagenarian siblings (n = 19), and their partners as controls (n = 18), were recruited from the Leiden Longevity Study. Participants (mean age 65 years) were studied in a controlled in‐hospital setting over a 24‐h period, receiving three isocaloric meals at 9:00 h, 12:00 h and 18:00 h. Lights were off between 23:00 h and 8:00 h. Serum total cholesterol (TC), HDL cholesterol (HDL‐C), non‐HDL‐C and triglycerides (TG) were determined every 30 min over a 24‐h period. Serum TC concentrations were higher during day than during night in offspring (5.2 vs. 4.7 mm, P < 0.001) and in controls (5.3 vs. 5.0 mm, P < 0.001). The difference in TC concentrations between day and night tended to be greater in offspring than in controls (0.5 vs. 0.3 mm, P = 0.109), reaching statistical significance in females (P = 0.045). Notably, the day–night serum differences in non‐HDL‐C were twofold greater in offspring than in controls (0.43 vs. 0.21 mm, P = 0.044) and most explicit in females (0.53 vs. 0.22, P = 0.078). We conclude that familial longevity is characterized by a high circadian rhythmicity of non‐HDL‐C in healthy elderly offspring from nonagenarian siblings.

Diurnal regulation of microsomal triglyceride transfer protein and plasma lipid levels

Plasma lipids are maintained within a narrow physiologic range and exhibit circadian rhythmicity. Plasma triglyceride and cholesterol levels were high in the night due to changes in apolipoprotein B-lipoproteins in ad libitum fed rats and mice maintained in a 12-h photoperiod. Absorption of [(3)H]triolein or [(3)H]cholesterol was higher at 2400 h than at 1200 h, indicating that intestinal lipoprotein production shows diurnal variation. Moreover, intestinal microsomal triglyceride transfer protein (MTP) activity, protein, mRNA, and gene transcription showed diurnal variations and were high at 2400 h. Similar to the small intestine, hepatic MTP activity, protein, and mRNA levels also changed significantly within a day. MTP was induced in fasted animals soon after refeeding. When mice were subjected to restricted feeding, MTP expression was high at the expected time of food availability. In contrast, extended exposures to light and dark completely abolished rhythmicity in MTP expression and plasma lipid levels. These studies show that MTP expression and plasma lipid undergo diurnal regulation and exhibit peaks and nadirs at similar times and suggest that diurnal modulation of MTP is a major determinant of daily changes in plasma lipids. Furthermore, environmental factors, such as food and light, play an important role in MTP regulation.

The human body may buffer small differences in meal size and timing during a 24-h wake period provided energy balance is maintained

Because approximately 20% of the work force in the industrialized world have irregular working hours, it is pertinent to study the consequences of eating at irregular, especially nighttime hours. We studied the postprandial responses during nocturnal fasting vs. eating throughout a 24-h wake period. Seven healthy males were studied twice in a crossover design. After a 6-d diet adjustment period [high fat diet, 45 energy percent (en%) fat, 40 en% carbohydrates)] with sleep from 2300 to 0700 h, the men were kept awake for 24 h at the metabolic ward and given either 6 isoenergetic meals, i.e., every 4 h (N-eat) or 4 isoenergetic meals from 0800 to 2000 h followed by a nocturnal fast (N-fast), with the same 24-h energy intake. Energy expenditure, substrate utilization, activity, heat release, body temperature and blood variables were measured over 24 h. Energy expenditure and blood glucose, triacylglycerol, insulin and glucagon concentrations were lower and nonesterified fatty acids concentrations were higher during the nocturnal fast than during nocturnal eating (P < 0.05); however, no 24-h differences between the protocols were apparent. Nocturnal fasting slightly altered the secretory patterns of the thyroid hormones and cortisol (P < 0.05). We found no clear indication that it would be more favorable to ingest few larger daytime meals than smaller meals throughout the 24-h period. The body seems to be able to buffer small differences in meal size and timing provided energy balance is maintained.

Different patterns of brisk walking are equally effective in decreasing postprandial lipaemia

OBJECTIVE

To compare the effects of different patterns of brisk walking on day-long plasma triacylglycerol concentrations in sedentary adults.

DESIGN

A three-trial, repeated measures design in which subjects were studied in the fasted state and throughout a day during which they consumed three standardized, mixed meals. On different occasions, subjects undertook no exercise (control), walked briskly for 10 min before each meal (short walks) or walked briskly for 30min before breakfast (long walk).

SUBJECTS

Seven postmenopausal sedentary women and three sedentary men aged between 34 and 66y, with body mass index between 24 and 35 kg/m2.

MEASUREMENTS

Plasma concentrations of triacylglycerol, non-esterified fatty acids, glucose and insulin, metabolic rate and whole-body substrate oxidation in the fasted state and at hourly intervals for 3 h after each meal.

RESULTS

Postprandial plasma triacylglycerol concentrations were lower (P= 0.009) during the walking trials than during the control trial (average values: control 2.08 +/- 0.28 mmol/l; short walks 1.83 +/- 0.22mmol/l; long walk 1.84 +/- 0.22mmol/l (mean+/-s.e.) but did not differ between the two patterns of walking. The difference between control and walking trials increased as successive meals were consumed (interaction of trial x meal P= 0.03). Plasma triacylglycerol concentration increased during the 3 h after breakfast, changed little after lunch and decreased after the evening meal (interaction of meal x time P=0.001). When both walking trials were treated as one condition, walking increased postprandial fat oxidation (average values: control, 0.066 +/- 0.009 g/min;walking 0.074 +/- 0.008 g/min; P < 0.01).

CONCLUSIONS

Thirty minutes of brisk walking, undertaken in one session or accumulated throughout a day, reduces postprandial plasma triacylglycerol concentrations and increases fat oxidation.

Chronic and postprandial responses of plasma insulin, glucose and lipids in volunteers given dietary fibre supplements

We questioned whether a dietary fibre supplement known to lower fasting plasma cholesterol concentrations can also lower the postprandial plasma cholesterol, glucose and insulin concentrations when it is administered just before a meal. Two studies were conducted in healthy middle-aged volunteers of both sexes in whom the fasting plasma total cholesterol concentrations were above normal. In the first study the dietary fibre treatments (2.2 g) were psyllium and a psyllium-citrus pectin mixture to which the subjects (four males, eight females) had no prior exposure. Controls received no supplement. The meals were high-fat breakfasts and lunches. In the second study the dietary fibre (6 g) was from sugar-beet root and the reference control was alpha-cellulose (2 g); the meal was of glucose. The volunteers (eight males, eight females) had prior exposure to the fibre supplements three times daily for 3 weeks. After adjustments for fasting values and changes in haemodilution, the psyllium and psyllium-citrus pectin mixture in the first experiment had no significant effects on the postprandial measurements of plasma glucose, insulin:glucose ratio, total-, LDL- and HDL-cholesterol, and triacylglycerol. By contrast, the sugar-beet fibre in the second study significantly decreased the area under the glucose response curve by 6.9%, the area under the insulin response curve was lower by 9.6%, although not significantly, and the post-glucose meal HDL-cholesterol concentration was significantly (12%) higher. Additionally, the 3-week treatment with sugar-beet fibre significantly lowered the fasting total- and LDL-cholesterol concentrations, by 8.5% and 9.6% respectively. We conclude that low doses of psyllium and citrus pectin at breakfast and lunch have no effects on the postprandial plasma measurements, but that sugar-beet fibre taken daily for 3 weeks affects both fasting and postprandial plasma metabolites favourably in these individuals with mildly increased risk of ischaemic heart disease. Further, we observed that small changes in haemodilution occur after meals, as indicated by plasma albumin concentration and packed cell volume. Underemphasis of the dietary fibre effects may occur when postprandial haemodilution is not taken into account.

Weight reduction increases adipose tissue lipoprotein lipase responsiveness in obese women

Lipoprotein lipase was measured in gluteal adipose tissue from nine obese (90.6 +/- 2.7 kg) women fasting and after the intravenous infusion of insulin and glucose before, immediately after, and 3 mo subsequent to a 14.0 +/- 1.8% (mean +/- SEM) weight reduction. Fasting adipose tissue lipoprotein lipase activity (ATLPL) decreased from 5.3 to 2.3 nEq FFA/10(6) cells per min (P less than 0.02) immediately after weight reduction, yet after weight maintenance, higher levels were again found (6.1 nEq FFA/10(6) cells per min). Although responsiveness of ATLPL to 40 mU/m2 per min of insulin infusion over 6 h was absent before weight loss, increases were seen immediately after weight loss (delta 0.8, P = 0.05) and more so (delta 7.7, P less than 0.01) after 3 mo. Moreover, whereas before weight loss the ATLPL response to ingested mixed meals (delta 0.9) was minimal, in the maintained reduced-obese state a marked increase was seen (delta 12.6, P = 0.02). Thus, because ATLPL is important to lipid filling in adipose tissue, the maintenance of high levels of fasting ATLPL and the increase in enzyme responsiveness in the reduced-obese state could play an important role in the resumption of the obese state, which so commonly follows weight reduction.

Diurnal variations of plasma lipids, tissue and plasma lipoprotein lipase, and VLDL secretion rates in the rat. A model for studies of VLDL metabolism

Circadian rhythms of plasma lipids and lipoproteins, lipoprotein lipase activities and VLDL secretion rates were studied in fed and food-deprived (12 h) male rats after a light/dark synchronization of 14 days. In ad libitum fed rats, a circadian rhythm of plasma triacylglycerol, blood glucose and liver glycogen was clearly identified. A rhythm was also identified for plasma cholesterol, but not phospholipids. The peak of plasma triacylglycerol occurred 2 h after the beginning of the light period (7.00 a.m.), and the nadir, 2 h after the beginning of the dark period (7.00 p.m.). The differences of plasma triacylglycerol at these two circadian stages were even more pronounced in food-deprived rats and were confined to the very-low-density lipoprotein (VLDL) fraction. Plasma post-heparin and heart and muscle lipoprotein lipase activities were 50-100% higher at 7.00 p.m., the time when plasma triacylglycerol were lowest, as compared to 7.00 a.m. Plasma post-heparin hepatic lipase and adipose tissue lipoprotein lipase activities, in contrast, did not change. VLDL secretion rates were somewhat higher at 7.00 a.m. compared to 7.00 p.m., but this difference was not significant. It is concluded that physiological variation of heart and muscle lipoprotein lipase together with small differences of VLDL secretion rates are responsible for normal range oscillations of plasma VLDL triacylglycerol levels.

Lipoprotein lipase activity and intramuscular triglyceride stores after long-term high-fat and high-carbohydrate diets in physically trained men

Men with regular physical training habits voluntarily increased their dietary fat intake from 43 to 54% of energy (E%) for four weeks. This was followed by a low-fat (29 E%), high-carbohydrate diet for another four weeks. During the high-fat diet period, the muscle lipoprotein lipase activity (LPLA) increased from 59 +/- 8 to 106 +/- 12 mU/g (mean +/- SE) (P less than 0.05). After the high-carbohydrate diet, LPLA was 57 +/- 16 mU/g, and unchanged relative to the pre-trial value. The triglyceride content in m. vastus lateralis increased from 30 +/- 4 to 47 +/- 9 mmol/kg d.w. (P less than 0.05; mean +/- SE) following the high-fat diet and to 41 +/- 8 following the high-carbohydrate diet. Neither of the diets affected the serum triglyceride and insulin concentrations, nor glucose, glycerol, beta-hydroxybutyrate, citrate and lactate levels in the blood. Nor did they alter enzyme activities in muscle used as markers for the oxidative (citrate synthase, beta-hydroxy-acyl CoA dehydrogenase) and glycolytic (glyceraldehyde phosphate dehydrogenase, lactate dehydrogenase) capacity. It is concluded that one month's adaptation to a high-fat diet results in increased muscle-LPL activity indicating a higher capacity for uptake of fatty acids from circulating serum triglycerides into the muscle cell in association with a greater capacity for triglyceride storage in the muscle. Under these conditions serum triglycerides were not decreased despite the increased muscle LPLA, and serum insulin variations could not explain the change in muscle LPLA.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic effects on plasma lipoprotein metabolism. Speculation on mechanisms of action

Recently, the effects of alpha-adrenergic and beta-adrenergic antagonists on plasma lipoprotein concentrations have been reported. Evidence from diverse lines of research has been brought together that suggests three potential mechanisms by which these antihypertensive agents affect lipoprotein metabolism. First, the known alterations in plasma triglyceride levels caused by adrenergic antagonists may be mediated through the activity of lipoprotein lipase. This enzyme, located on the capillary endothelium of adipose tissue and skeletal muscle, catabolizes chylomicrons and very-low-density lipoproteins. Catecholamine-induced changes in precapillary sphincter tone could affect the delivery of triglyceride-rich lipoproteins to endothelial lipase, cause changes in capillary endothelial surface area for lipase-binding sites, and/or modulate the synthesis of lipoprotein lipase by adipocytes and myocytes. Since high-density lipoprotein levels increase by taking up components of chylomicrons released by lipoprotein lipase, these pathways might explain the adrenergic-induced changes in high-density lipoprotein that are reciprocal to those in plasma triglycerides. Second, hepatic production of very-low-density lipoproteins might be affected by adrenergic-induced changes in insulin release. Decreased insulin release may direct glucose metabolites from adipocytes to hepatocytes for lipogenesis. Catecholamines and other glucoregulatory hormones are known to alter hepatic cholesterol synthesis and secretion of very-low-density lipoproteins. These observations also suggest that dietary carbohydrate and fat might modulate adrenergic influences on lipoprotein metabolism by other than classic means. Third, as suggested by studies of cultured fibroblasts, alpha-adrenergic antagonists may increase receptor-mediated catabolism of low-density lipoprotein.

Diurnal changes in serum triglycerides as related to changes in lipolytic enzymes, lipoproteins and hormones in patients with primary endogenous hypertriglyceridaemia on a carbohydrate-rich diet

Parameters of diurnal triglyceride (TG) metabolism were investigated in 5 subjects with primary endogenous hypertriglyceridaemia and compared with those of normal subjects studied previously. The patients were in a steady state on a carbohydrate-rich diet (meals at 9.00, 13.00 and 17.00 h). Serum TG showed a wavelike pattern with a maximum at around 17.00 h. Post-heparin lipoprotein lipase (LPL) activity in the fasting state was not different from that in normals, but failed to show the normal increase in the fed state (16.30 h). This was due to the inability of patients to increase their adipose tissue (AT)-LPL activity in the course of the day. AT-LPL activity was throughout the day lower than in normal subjects. Skeletal muscle LPL activity was low and showed no diurnal change, equalling our findings in normal subjects. Low density lipoprotein cholesterol and high density lipoprotein (HDL) cholesterol concentrations showed no diurnal change. However, HDL phospholipids increased significantly in the course of the day.

Diurnal course of the suitability of serum samples obtained from normal subjects and from patients with primary endogenous hypertriglyceridaemia to serve as substrate for milk lipoprotein lipase

Serum samples, obtained at various times of the day from four normal subjects and five patients with primary endogenous hypertriglyceridaemia on a carbohydrate-rich diet (meals at 09.00, 13.00 and 17.00 h), were incubated with cows' milk lipoprotein lipase (LPL) to investigate the susceptibility of the triglyceride(TG)-rich lipoprotein fraction to hydrolysis. Mean KM and vmax of this reaction increased in patients between 09.00 and 13.00 h (p = 0.016 and 0.015, respectively) and decreased again thereafter, whereas no diurnal change was observed in normal subjects (p = 0.31 and 0.27, respectively). At 13.00 and 23.00 h patients showed a higher mean KM (p less than 0.02) at 23.00 h a higher mean vmax (p less than 0.03) than normal subjects. When the data were expressed as reaction rates (FFA formation rates) at TG-concentrations in the in vivo range, neither group showed a diurnal difference. Patients showed a significantly lower reaction rate than normal subjects only at 13.00 h at TG concentrations below 3 mmol X 1-1. This suggests that a defect in the suitability of TG-rich particles to serve as a substrate for LPL is not likely to play a role in the development of hypertriglyceridaemia.